The return-to-zero (RZ) pulse format for optical signals is known to offer performance advantages such as high receiver sensitivity and high immunity to inter-symbol interference as compared to non-return-to-zero (NRZ) formats. DPSK optical transmission has also been found to provide performance advantages when coupled with balanced detection, such as superior receiver sensitivity, high tolerance to some major nonlinear effects in high-speed transmissions, and high tolerance to coherent crosstalk. RZ-DPSK has accordingly become a modulation format of choice for high-capacity, long-haul transmissions.
Proposed schemes for the generation of chirp-free RZ-DPSK signals require both a Mach-Zehnder modulator (MZM) for pulse carving and another MZM for exact phase modulation. Such a format is discussed in A. H. Gnauck, “40-Gb/s RZ-differential phase shift keyed transmission,” Optical Fiber Communication Conference 2003 (OFC'03), Paper ThE1 which is incorporated herein by reference.
A scheme for generating RZ on-off-keyed (RZ-OOK) signals using a single MZM and a differential amplifier has been proposed and is discussed in Y. H. Kao et al, “10 Gb/s soliton generation for ULH transmission using a wideband GaAs pHemt amplifier,” OFC'03, Paper FF6, which is incorporated herein by reference. However, such a scheme cannot be used for the generation of RZ-DPSK signals because it is not capable of generating the three states, ‘1’, ‘0’, and ‘−1’, in the optical field domain, of an RZ-DPSK signal.
A scheme for the introduction of chirp in RZ-OOK optical signals (i.e. CRZ-OOK signals) has been proposed in B. Bakhshi, et al, “Comparison of CRZ, RZ, and NRZ modulation formats in a 64×12.3 Gb/s WDM transmission experiment over 9000 km,” in Proc. OFC'01, paper WF4, 2001, which is incorporated herein by reference. The introduction of chirp in RZ-OOK optical signals was found to provide increased resistance to nonlinear effects in long-haul 10 Gb/s transmissions. However, such a scheme requires the use of an additional phase modulator to introduce chirp into the signal, which increases the complexity and cost of the transmitter.